This invention relates to hollow containers with inert and/or impermeable inner surfaces and, more particularly, to such containers produced by the application of a thin coating either directly to the container preform or to the core rods of the preform injection machine.
Plastic containers have been replacing glass in many applications where easy handling, low weight and non-breakability are needed. To date, polymers have had varying degrees of inertness to the packaged content which differ from the inertness of glass. In the case of plastic food packages, surface inertness helps diminish potential desorption of packaging material components into the food, to prevent flavor absorption, to avoid loss of food constituents through the package walls and to avoid ingress of air or other substances from outside the package.
Refillable plastic packages add a further dimension to inertness requirements because these packages must withstand washing and refilling. Such containers should not absorb contact materials including, inter alia, washing agents and materials stored in the container.
Packages for carbonated beverages are pressurized and must withstand considerable mechanical stress in handling. It is therefore difficult for a single material to provide the necessary mechanical stability and the required inertness.
Current plastic packages for carbonated beverages consist of either a single material, e.g. polyethylene terephthalate (PET), or of multi-layer structures where the middle layers normally provide the barrier properties while the outer layers provide the required mechanical strength.
Multi-layer containers are produced either by coinjection or coextrusion but these processes restrict choice of materials and cannot provide a very thin inner layer.
Therefore, a plastic container with an impermeable, dense, xe2x80x9cglass-likexe2x80x9d inner surface cannot be produced by conventional methods because these limit the options for the internal surface. Where a plastic, such as high-crystalline PET has good barrier properties but poor transparency, a very thin inner layer is needed so that the transparency of the container as a whole is not impaired. Where a plastic, intended as inner layer, has a different glass transition temperature than the main container material, it cannot be blow-molded unless the inner layer is very thin and can be subjected to individual heat treatment. And, where a barrier plastic has residual monomers or depolymerization by-products, such as acetaldehyde for PET, these can be extracted or deaerated from a very thin layer but not from a thick layer. Accordingly, more polymer options are possible with very thin layer structures.
Recycling is yet another dimension insofar as mass-produced packages are concerned. The reuse of recycled plastic for same purpose, that is to produce new containers (xe2x80x9cclosed-loopxe2x80x9d recycling) is an issue which has attracted much attention, and for PET, this has been achieved to-date by depolymerizing the recycled material in order to free it of all trace contaminants which might otherwise migrate and come in contact with the container content. An impermeable inner layer, which is the purpose of this invention, would enable recycled material to be reused directly for new containers i.e. without special treatment such as depolymerization, since traces of foreign substances could no longer contact the container""s content. This would simplify the xe2x80x9cclosed-loopxe2x80x9d recycling process considerably by obviating the need for depolymerization.
Furthermore, recyclability within established recycling systems, both xe2x80x9copen-loopxe2x80x9d, i.e. recycling for other uses, or xe2x80x9cclosed-loopxe2x80x9d, i.e. reuse for same purpose, is necessary for any mass-produced package. In xe2x80x9copen-loopxe2x80x9d systems, the normal method is to separate, clean and chop up the plastic into small flakes. The flake is then either melted and used for molding other objects, or for fibre production. For this form of recycling, it is important to note that any contaminant to the main plastic, such as a coating, should effectively be present in negligible quantities and, preferably, be solid and insoluble within the molten plastic so that it can be filtered off prior to sensitive applications, such as fibre-production. PET is also recycled in xe2x80x9cclosed-loopxe2x80x9d by depolymerization and it is important that the coating material should be unchanged by this process, be insoluble in the monomers resulting from the process, and be easily separable from these monomers. With a correct choice of material, a thin, inner layer can fulfill these criteria.
Finally, since one option available when using a single very thin layer is to use a highly crystalline version of the same polymer as is used in the main part of the container, e.g. highly crystalline PET in PET containers, the inner layer is virtually the same as the outer and recycling presents no problems.
As a result of these inherent limitations, current technology cannot now produce containers with a high barrier inert inner layer having a good appearance because it forces compromises which detract from the desired end result which is an improved beverage container.
Accordingly, it is the primary object of this invention to fabricate a very thin inner layer composed of a polymer chosen for its barrier and/or inertness properties within a container constructed mainly from another polymer, or same polymer in a significantly more crystalline state, or a structure of polymers.
This and other objects are fulfilled by a method and apparatus for forming a thin inner polymer layer within the preform, then further reducing the thickness of this layer when the container is subsequently formed by stretch blowing. The inner layer on the preform is produced either by a controlled coating method involving coating of the injection mold core rod prior to injection molding or by a coating applied directly to the reheated surface of the preform after injection molding. This enables both inner and main layers to be brought together at a time when the interface between them is molten. Such fabrication offers resistance to subsequent delamination. Additionally, this enables one to use a conventional tie layer, if necessary, and also enables the layers to be combined, where appropriate, without melting the layer interface. Furthermore, this enables use of an inner layer whose glass transition temperature is different from that of the main material in the preform.
Since a very thin layer of polymer is used, the problems of residual monomers, or other extractables, can be resolved by conventional means, such as by deaeration or extraction which is at present far less practicable when the material layer used is thicker, or when the material in contact with the food does not present a good barrier to migration from other layers within the container wall structure. Finally, since some barrier materials have poor transparency, the use of a very thin layer enables transparency problems to be avoided or at least reduced. This invention therefore provides greater flexibility in selecting the material for the inner contact layer of a container on the basis of its barrier properties and inertness, thereby avoiding undesirable compromises which are imposed by the current technology.